build-localimage should document minimum version to produce compatible base image

It appears that a container built with singularity 2.1.2 does not serve as a proper base image to build a new container with 2.4.2 since the environment is subsequently gone.

The page at http://singularity.lbl.gov/build-localimage should probably mention this, but it would be even better if singularity could properly support building off old containers.